Adjacent Channel Splatter

The ratio of power transmitted within the assigned channel to power leaked into adjacent channels during the transient. A lower ratio indicates poor filtering or aggressive power amplifier ramping. This metric is calculated by integrating the short-time Fourier transform over the burst onset period and comparing the energy in the primary channel to the energy in the first and second adjacent channels. Power amplifier non-linearity during the ramp-up is the primary physical cause, as the transistor operates in its non-linear region before stabilizing at its quiescent bias point.

The time-varying power envelope measured in the adjacent channels during the transient. Unlike steady-state adjacent channel leakage ratio, this profile is dynamic and reveals the switching speed of the transmitter's power control loop. Key features include:

• Peak splatter amplitude: The maximum instantaneous power in the adjacent channel
• Splatter duration: The time interval over which the adjacent channel power exceeds a defined threshold
• Decay slope: The rate at which splatter power diminishes as the amplifier settles into linear operation

The damped oscillation visible in the adjacent channel power envelope caused by the impulse response of the transmitter's output filter. When the power amplifier turns on abruptly, the sharp edge excites the reactive components in the matching network and bandpass filter. The resulting ringing leaks into adjacent channels with a characteristic resonant frequency and exponential decay constant. The Q-factor of this ringing is a direct fingerprint of the filter's component tolerances and parasitic elements.

The difference in splatter power between the upper and lower adjacent channels during the transient. This asymmetry is caused by AM-to-PM conversion in the power amplifier, where amplitude variations during the ramp-up induce phase modulation that skews the spectrum. The degree of asymmetry reveals:

• The memory effect of the amplifier's bias network
• The even-order distortion characteristics of the active device
• The load mismatch at the amplifier's output port

The ratio of peak adjacent channel power during the transient to the steady-state adjacent channel leakage ratio. A high ratio indicates a transmitter with poor transient power control despite acceptable continuous-wave linearity. This metric is particularly valuable for fingerprinting because it isolates the dynamic behavior of the automatic power control loop and the gate bias sequencing circuit, which are highly component-specific and difficult to calibrate out.

The instantaneous bandwidth of the spectral splatter as it spreads beyond the assigned channel. During the first microseconds of turn-on, the splatter can occupy a bandwidth several times wider than the steady-state modulated signal. This transient bandwidth expansion is caused by the high-frequency Fourier components of the sharp ramp edge. The specific shape of the spectral roll-off in the adjacent channels reveals the order of the reconstruction filter and the slew rate of the digital-to-analog converter driving the modulator.

The parent phenomenon of Adjacent Channel Splatter, referring to the broadband spectral noise generated by the rapid switching of a transmitter during turn-on or turn-off. This splatter is a direct consequence of the fast rise-time of the signal envelope, which, by the Fourier transform, produces wideband frequency components. The specific energy that falls into neighboring channels is what defines the adjacent channel splatter metric, making it a key indicator of a transmitter's switching speed and power amplifier linearity.

A historical term originating from telegraphy that is now applied to modern transient-induced spectral artifacts. It originally described the interference caused by the abrupt make/break of a telegraph key, which generated wideband 'clicks' in nearby receivers. In modern contexts, key-click analysis refers to the study of the spectral sidebands created by the sharp rising and falling edges of on-off keyed or burst-mode transmissions, directly quantifying the energy that manifests as adjacent channel splatter.

The statistical distribution of the measured 10% to 90% rise time across multiple burst transmissions from the same device. A faster, more consistent rise time creates a sharper edge, which concentrates more energy into a wider spectral bandwidth, directly increasing adjacent channel splatter. The variance itself is a unique hardware fingerprint, reflecting the stochastic nature of the power-up sequence and the slew rate of the power amplifier.

The unique pattern of electromagnetic interference radiated or conducted from a device during the switching transient. Adjacent channel splatter is the manifestation of this EMI within the frequency domain of a neighboring communication channel. This signature is a byproduct of rapid current changes (di/dt) in circuit loops and can be analyzed to identify a device. Key components include:

• Conducted emissions on power and signal lines
• Radiated emissions from PCB traces and antennas
• Parasitic coupling between components

The non-linear amplitude and phase distortion generated by the power amplifier (PA) when it is driven through its non-linear region during the rapid ramp-up of the signal envelope. This is a primary root cause of adjacent channel splatter. As the PA transitions from an off-state to saturation, its gain compresses and it generates intermodulation products and spectral regrowth, which spill energy into adjacent channels before the amplifier settles into its linear operating mode.

The complete time-domain response of the phase-locked loop (PLL) as it acquires lock on the target frequency after power-up. During this period, the carrier frequency is not stable and can exhibit frequency overshoot and ringing. This dynamic behavior frequency-modulates the signal, broadening its instantaneous spectrum and contributing significantly to transient spectral splatter. The loop filter's damping factor, a component-specific value, dictates the severity of this effect.